The present invention relates to a process for the production of xylitol and/or ethanol from lignocellulose-containing material.
Xylitol is a naturally occurring sugar alcohol which is formed in the reduction reaction of xylose and which corresponds to “normal” sugar in sweetness and low in caloric content (i.e. 2.4 kcal/g). Xylitol is found in small quantities in many fruits and vegetables and is also produced in the human body as a normal metabolic product. Xylitol is a very good special sweetener in different connections on account of its certain metabolic, dental and technical properties. Desirably, the xylitol metabolism is independent of the insulin metabolism, and therefore also diabetics can use xylitol. Xylitol also has a retarding effect on the bowel and may have utility in reducing diets. Furthermore, it has been found that xylitol does not cause caries but has a anti-cariogenic effect.
Despite the many advantages of xylitol, its use has been rather restricted. The reason for this is the relatively high price of xylitol, which in turn is a result of the difficulties of producing xylitol on a larger scale.
Xylitol has earlier been produced from xylan-containing materials by hydrolysis, in which process a monosaccharide mixture containing e.g. xylose is obtained. Xylose is then converted to xylitol, generally in the presence of a nickel catalyst, such as Raney nickel. A number of processes for the production of xylose and/or xylitol from a xylan-containing material have been described in the literature in this field. As examples may be mentioned U.S. Pat. No. 3,764,408 (Jaffe at al.), U.S. Pat. No. 4,066,711 (Melaja at al.), U.S. Pat. No. 4,075,406 (Melaja at al.), U.S. Pat. No. 4,008,285 (Melaja at al.) and U.S. Pat. No. 3,586,537 (Steiner at al.).
These prior processes are all multi-step processes which are relatively costly and often have inadequate efficiency. The greatest problems reside in the effective, high yield separation of xylose and/or xylitol from polyols and other hydrolysis and conversion products and the use of the by-products which are produced in large quantities in the process. Purification of the preceding can be very exacting because catalysts used in the reduction reaction of xylose are very sensitive. The purity of the final product often depends on the extent that the xylitol can be separated from the other products produced in the reduction reaction.
It is known that several yeast strains produce reductase enzymes which catalyze the reduction of sugars into corresponding sugar alcohols. Certain Candida strains have been reported to produce xylitol from xylose (Ditzelmullez, G. at al.: FEMS Microbiology Letters 25 (1985), pp. 195–198, Kitpreechavanich, M. at al.: Biotechnology Letters Vol. 6 (1984), pp. 651–656, Gong, C-S. at al.: Bioztechnology Letters Vol. 3 (1981), pp. 130–135). However, these studies have been carried out on a laboratory scale only, and the literature in this field has not disclosed processes wherein pure xylitol is separated from the fermentation product.
The Applicants' copending U.S. patent application Ser. No. 297,791 filed on Jan. 17, 1989, now U.S. Pat. No. 5,081,026, and published as WO-A-9008193, describes a process for the production of pure xylitol from plant material using chromatographic separation following hydrolysis and fermentation. However, in this process the majority of the raw material can be lost as a worthless waste material. If a greater part of the raw materials could be converted to commercial products, this would essentially improve the economy of the overall process.
Ethanol is a well-known compound which has a wide use. Ethanol has attracted interest as an alternative liquid fuel. If the ethanol production process only uses energy from renewable energy sources, no net carbon dioxide is added to the atmosphere, making ethanol an environmentally beneficial energy source.
It is known that ethanol can be produced from cellulose and hemicellulose by fermenting with a suitable yeast strain. The production of ethanol from D-xylose has been described in U.S. Pat. No. 4,368,268 (C-S. Gong), which is directed to the manufacturing of mutants that produce ethanol in high yields, and in Biotechnology and Bioengineering Symp. 12 (1982), pp. 91102, McCracken, L. & Gong, C-S., which is directed to fermentation with thermotolerant yeasts.
Ethanol production from lignocellulosic material can comprise the following steps: (1) degradation of the lignocellulosic structure to a fermentable substrate, (2) fermentation of the fermentable substrate, and (3) distillation of the fermentation broth to obtain ethanol.
In the past, there have been problems encountered in the efficient conversion of the lignocellulosic hydrolysates to ethanol. First, after pretreatment, the hydrolysate contains not only fermentable sugars, but also a broad range of compounds which often have inhibitory effects in the microorganisms used for fermentation. The composition of these compounds depends upon the type of lignocellulosic material used and the chemistry and nature of the pretreatment process. Second, the hemicellulose hydrolysates contain not only hexoses but also pentoses. The pentose fraction in hemicellulose comprises mainly xylose, but depending on the raw material origin, the arabinose fraction may be substantial. While some hexoses can readily be fermented, pentoses are more difficult to ferment.
Lignocellulosic materials are composed of mainly cellulose, hemicellulose, and lignin. Cellulose is a linear, crystalline polymer of β-D-glucose units. The structure is rigid and harsh treatment is usually required to break down cellulose. Hemicellulose has usually as a main component linear and branched heteropolymers of L-arabinose, D-galactose, D-glucose, D-mannose, D-xylose and L-rhamnose. The composition of hemicellulose varies with the origin of the lignocellulosic material. The structure is not at least totally crystalline and is therefore usually easier to hydrolyze than cellulose. Examples of lignocellulosic materials considered for ethanol production are hardwood, softwood, forestry residues, agricultural residues, and municipal solid waste (MSW). Both cellulose and hemicellulose can be used for ethanol production. The pentose content in the raw material is of importance as pentoses are often difficult to ferment to ethanol. The pentose content can comprise 6 –28% of the total dry matter. To achieve maximum ethanol yield, all monosaccharides should be fermented. Softwood hemicellulose contains a high proportion of mannose and more galactose and glucose than hardwood hemicellulose whereas hardwood hemicellulose usually contains a higher proportion of pentoses like D-xylose and L-arabinose.
The degradation of the lignocellulosic structure often requires many steps. The first step can comprise prehydrolysis in which the hemicellulose structure is broken down. The second step can comprise the hydrolysis of the cellulose fraction in which lignin will remain as a solid by-product. The two hydrolyzed streams can be fermented to ethanol either together or separately, whereafter they can be mixed together and distilled.
As previously discussed, during the degradation of the lignocellulosic structure, not only fermentable sugars are released, but a broad range of compounds, some of which can inhibit the effectiveness of the microorganism used for fermenting. The amount and nature of inhibiting compounds depends on the raw material, the prehydrolysis and hydrolysis procedures, and the extent of recirculation in the process. Fermentation inhibitors in lignocellulosic hydrolysates can be divided into several groups depending on their origin. Substances released during prehydrolysis and hydrolysis include acetic acid, which is released when the hemicellulose structure is degraded and extractives. The extractives can comprise terpenes, alcohols, and aromatic compounds such as tannins. The inhibitory effect of acetic acid is pH dependent. The fermentability of a lignocellulosic hydrolysate can be improved by raising the pH. Furthermore, a group of inhibitors, such as furfural, 5-hydroxymethyl furfural, laevulinic acid, formic acid, and humic substances are often produced as by-products in prehydrolysis and hydrolysis due to the degradation of sugars. Moreover, lignin degradation products are often produced in prehydrolysis and hydrolysis. This group of inhibitors includes a wide range of aromatic and polyaromatic compounds with a variety of substituents. Also, products of the fermentation process, such as ethanol, acetic acid, glycerol, and lactic acid, inhibit the microorganism. The influence of these compounds will be especially evident in recirculation systems. Furthermore, metals released from the equipment and additive such as sulfur dioxide (SO2) can also inhibit fermentation.
It is, therefore, desirable to provide an improved method to produce ethanol, as well as perhaps other important products, such as xylitol, from lignocellulose-containing material in biomass.